ObjectivesThis study aims to evaluate the attenuation of renal calculi measured by Hounsfield unit (HU) by noncontrast spiral computerized tomography (NCSCT) as a predictor of calculus fragmentation by extracorporeal shock wave lithotripsy (SWL).BackgroundThe outcome of extracorporeal SWL is measured in terms of stone fragmentation and clearance. Some authors have suggested that HU of renal calculi by NCSCT may predict stone-free rates after extracorporeal SWL.Patients and methodsThis prospective nonrandomized open study included 100 patients with renal stone up to 20 mm in size. Stone attenuation was measured by HU on NCSCT. Patients were grouped according to stone attenuation as group (1), less than 500 HU, (2), 500-1000 HU, and (3), greater than 1000 HU. Patients were treated subsequently with extracorporeal SWL. The outcome was categorized as stone free, clinically insignificant stone fragments, and residual fragments more than 3 mm.ResultsThe rate of stone fragmentation was 100% (41 of 41 cases) in group 1, 95.7% (44 of 46) in group 2, and 0% (0 of 13) in group 3. A statistically significant association was found between SWL treatment outcome and stone density. When we correlated the absolute stone HU measured with the number of shock waves required for complete fragmentation, we found that the mean number of shock waves was 248 122 in group 1, 334 826 in group 2, and 726 077 in group 3 (P < 0.001).ConclusionStone attenuation measured by HU by NCSCT is a predictor of outcome of SWL and suspected number of shock waves required for fragmentation.

The outcome of shock wave lithotripsy (SWL) is measured in terms of stone fragmentation and clearance. Failure of SWL results in unnecessary exposure of renal parenchyma to shock waves and its possible complications. Invariably, alternative treatments are then needed, incurring additional medical expenses [1] .

Because disintegration is the first step in the treatment of renal stones by SWL, the magnitude of response of a calculus to disintegration (i.e. stone fragility) should be considered before using SWL. It is often not possible to predict whether a given stone is amenable to fragmentation by shock waves before starting treatment; however, because there are many factors that affect stone fragility, such as size and composition [2] .

Therefore, from the inception of SWL, factors predicting its outcome have been studied. A number of stone characteristics such as fragility, size, location, and composition are known to affect outcome [3] .

Hounsfield unit (HU) measurement of urinary calculi on pretreatment NCCT may predict the stone-free rate; this information may be potentially beneficial for selection of the preferred treatment option for patients with urinary calculi [5] .

Patients and methods

This is a prospective study that was carried out at the urology department in Menoufia University Hospital between December 2010 and February 2012. The study included 100 patients who presented with renal stones.

Preoperative evaluation

Our study included 100 patients with symptomatic renal stone measured between 5 and 20 mm, either radiolucent or radiopaque.

Patients with multiple renal stones were excluded. In addition, patients with stone less than 5 mm or larger than 20 mm in size, and those with elevated serum creatinine levels ( > 2 mg/dl) and bleeding diathesis were also excluded.

All selected patients were subjected to assessment of complete history and full physical, urological examination, hematological, biochemical, and radiological evaluations.

NCSCT was performed using a multidetector row helical CT scanner (Toshiba Asteion, Japan) for all patients. The images were obtained using the high-quality mode at 200 mA, 120 kV, and 5 mm collimation reconstructed at 3 mm. The postscanning bone window protocol was used to measure the stone attenuation value (HU) for the determination of stone density.

Patients were grouped according to stone density into three groups:

Group 1: stones less than 500 HU, group 2: stones between 500 and 1000 HU, and group 3: stones more than 1000 HU.

Operative procedure

Patients were treated by extracorporeal SWL using the electrohydraulic Lithotriptor MT1-RX (BMA for design and industry; Egypt) under spinal or general anesthesia. Localization was performed using fluoroscopy for both radiopaque and radiolucent stones using iodinated contrast agents for radiolucent stones.

Stones are exposed to shock waves ranging in intensity from 1 to 21 kV, starting with 200 shock waves at 12 kV, followed by 22 kV, with a maximum number of 3000 shocks per session.

The treatment should be started on a lower energy setting with a step-wise power ramping [6] .

Careful control of pain during treatment is necessary to limit pain-induced movements and excessive respiratory excursions.

A change in stone size, outline, or separation indicates fragmentation.

Another SWL session was performed after 3 weeks if follow-up plain KUB (plain abdominal film of the kidneys, ureters, and bladder) showed significant residual fragments ( > 3 mm for radiopaque stones where radiolucence was monitored by ultrasound).

Postoperative follow-up

Patients were instructed to collect post-SWL urine sample for analysis of stone fragments.

The SWL result was considered successful with complete clearance of the stone (stone free) or asymptomatic residual fragments less than or equal to 3 mm in size (clinically insignificant stone fragments) and considered a failure with residual fragments more than 3 mm in size after three SWL sessions (residual stone) [7] .

Repeat treatment was carried out if inadequate fragmentation of the stone was observed.

Statistical analysis

Data were statistically described in terms of mean ± SD and percentages when appropriate. For comparison of categorical data, the χ2 -test was used and the student t-test was used for numerical data. P values less than 0.05 were considered statistically significant. All statistical calculations were carried out using the computer program: statistical package for social science version 15 (SPSS Inc., Chicago, Illinois, USA) for Microsoft Windows.

Results

The mean HU measured by NCSCT before SWL treatment was 625.62 ± 310.409 HU. There was no hydronephrosis in 40% of patients; 53% of patients had mild hydronephrosis and 7% of the patients had moderate hydronephrosis. Fifty-eight patients were stone free, 27 had clinically insignificant stone fragments ( < 3 mm), and 15 had significant residual stones.

Of the 100 patients, 41 (41%) were in group 1, 46 (46%) were in group 2, and 13 (13%) were in group 3. The rate of stone fragmentation was 100% (41 of 41 cases) in group 1, 95.7% (44 of 46) in group 2, and 0% (0 of 13) in group 3 (P < 0.001, Graph 1) [Additional file 1].

We found that the mean number of shock waves was 248 122 in group 1, 334 826 in group 2, and 726 077 in group 3. The success rate for stones with density greater than 1000 HU was significantly lower than that for stones with a density of less than 1000 HU. A χ2 -test analysis showed a statistically significant association between SWL treatment outcome and stone density (χ2 = 94, d.f. = 4, P < 0.001, Graph 2) [Additional file 2].

The analysis of stone type in relation to the fragmentation rate showed that determination of HU aids differentiation of urate stones from other types of stones. Thus, calcium oxalate and calcium phosphate had the highest rates of failure whereas urate stones had the highest rates of success.

Discussion

The impact of stone attenuation on SWL outcome has been studied by several investigators [8] . Moreover, because there is marked overlap between attenuation values of different stone classes, stone composition cannot be accurately predicted before the retrieved stones are analyzed. Nakada et al. [9] could differentiate between uric acid and calcium oxalate monohydrate stones using peak attenuation measurement, whereas Sheir et al.[10] could differentiate only between pure stone classes. Thus, determination of stone composition before treatment is difficult and may not be sufficient to allow prediction of the response to SWL. Therefore, pre-SWL radiographic examination should focus on those radiological stone characteristics that influence SWL outcome rather than on stone composition [11] .

Therefore, from the inception of SWL, factors predicting its outcome have been studied. A number of stone characteristics such as fragility, size, location, and composition are known to affect outcome [3] .

The determination of stone density obtained on NCSCT is easy, objective, reliable, and reproducible. NCSCT is noninvasive and provides better density discrimination than conventional radiography; it can be used to detect a density difference of 0.5%, whereas plain radiography requires a density difference of 5% [12] .

A correlation between stone attenuation and stone fragility was first found in vitro. As the attenuation value of calcium stones increases, a greater number of shock waves are needed for fragmentation [13] .

In our study, patients were grouped according to stone density into three groups.

Group 1: stones less than 500 HU, group 2: stones between 500 and 1000 HU, and group 3: stones more than 1000 HU.

It was found that the rate of stone clearance was 100% (27 of 27 cases) in group 1, 96% (25 of 26) in group 2, and 0% (0 of 7) in group 3. When we correlated the absolute stone HU measured with the number of shock waves required for complete fragmentation, we found that the mean number of shock waves was 248 122 in group 1, 334 826 in group 2, and 726 077 in group 3.

A study by Joseph et al.[14] of 65 patients treated with SWL (Dornier Medical Systems Inc., Marietta, Georgia, USA) showed that stones with densities less than 500 HU have a 94% clearance rate and required a median of 2800 shock waves, patients with stone densities of 500-1000 HU have a 76% clearance rate and required a median of 3700 shock waves, and patients with stone densities more than 1000 HU have a 42% clearance rate and required a median of 7800 shock waves.

The patients were further analyzed by dividing them into three groups according to stone density. Low-density group: patients with stone densities of less than 500 HU, medium-density group: patients with stone densities of 500-1000, and high-density group: patients with stone densities of more than 1000. SWL treatment outcomes, according to stone density levels, showed a high success rate in the low-density group (94%).

Gupta et al. [18] evaluated 112 patients with solitary renal and upper ureteric calculi of 0.5-2 cm, undergoing SWL (Siemens Lithostar Shock Wave System C; Erlangen, Germany). They found that there was a linear relationship between the calculi density and number of SWL sessions required. Patients with calculi of 750 HU or less and a diameter less than 1.1 cm needed three or fewer SWL sessions and the clearance rate was 90%, whereas patients with calculi of more than 750 HU and a diameter of more than 1.1 cm needed three or more ESWL sessions and the clearance rate was 60%.